RegisterClass

RegisterClass is a fundamental function in the Microsoft Windows operating system's graphical user interface (GUI) programming. It is part of the Windows API and is used to inform the operating system about a new window class. A window class is a template that defines the properties and behavior of a window, such as its appearance, style, and how it handles messages.

When a program needs to create a window, it first defines a window class using the WNDCLASS structure. This structure contains information like the window's icon, cursor, background color, and the window procedure (a callback function that processes messages sent to the window). After filling out the WNDCLASS structure, the program calls the RegisterClass function, passing a pointer to the structure. This call registers the class with the operating system, making it available for creating windows.

Once a window class is registered, a program can create multiple instances of windows based on that class using the CreateWindowEx function. Each window created from the same class will share common characteristics defined in the class template. If RegisterClass is called with a class name that has already been registered, the function fails. This mechanism ensures that each unique window class is defined only once, preventing conflicts and maintaining a structured approach to window management within the Windows environment.